Memory T cells are important for protection against repeat exposures to infectious agents. In man, how memory T cells are generated or maintained during the adult years, and how the number and type of T cell changes with pathogen re-exposure are still poorly understood. We address these issues in a detailed manner in this proposal. The initial system we will investigate is the memory response of cytotoxic T cells to influenza matrix protein restricted by the HLA-A2 molecule. We have shown that the memory T cells comprise a diverse repertoire constituted of a large number of T cells and their progeny. These T cells all use the same T cell receptor (TCR) beta-chain gene and show a conserved amino acid sequence in the CDR3. The repertoire is composed of many T cells that are present at low numbers and a few that are present at higher frequencies. Our hypothesis is that robust memory T cell repertoires are complex and this complexity results from the manner in which the repertoire is selected. The experiments proposed in this Project benefit directly from the Sampling Core providing PBMC from HLA-A2 blood donors at multiple times. We propose the following Aims: 1. Analyze the response to influenza M1 + HLA-A2 across two cohorts of blood donors, all aged 36-50, all age groups in the HLA typed donor pool. This will provide a description of repertoire diversity across a population of individuals. 2. Test the ability of the adult memory repertoire to respond to related antigenic peptide. This tests the hypothesis that a robust system will be able to recognize related antigens that result from an attempt at viral evasion or infection with a related virus. 3. Perform longitudinal analyses on a subset of the donor pool, correlating changes with risk of exposure to influenza. These studies should show how the repertoire changes with re-exposure to pathogen 4. Determine to what extent conclusions reached are a general phenomenon by testing another influenza-derived HLA-A2 restricted peptide and an influenza derived peptide restricted by a different HLA molecule, in this case HLA-A11. The completion of these aims should provide us with a detailed quantitative and qualitative description of how robust memory T cells are maintained in adults and how they change with antigen exposure. This should lead to a better understanding of how memory T cells evolve and how they may be fostered.